Electrolyte

ABSTRACT

An electrolyte for a battery comprises LiBOB salt in gamma butyrolactone and a low viscosity solvent. The low viscosity solvent may comprise a nitrile, an ether, a linear carbonate, or a linear ester. This electrolyte is suitable for use in lithium ion batteries having graphite negative electrodes. Batteries using this electrolyte have high conductivity, low polarization, and high discharge capacity.

REFERENCE TO PRIOR FILED APPLICATIONS

This application claims the benefit of U.S. Provisional Application No.60/408,100 filed Sep. 3, 2002.

TECHNICAL FIELD

This invention relates to an electrolyte and more particularly to anelectrolyte for use in a battery.

BACKGROUND

An effective solid electrolyte layer (SEI) must be created at thesurface of a graphite negative electrode of a battery in order to keepthe electrolyte from decomposing. Various electrolytes comprisingcertain combinations of salts and solvents produce SEI layers of variousqualities. Typical lithium ion batteries use an electrolyte comprisingLiPF₆ in a carbonate solvent, with 1.2-M LiPF₆ in ethylene carbonate(EC): diethyl carbonate (DEC) being typical in the battery industry. ECis solid at room temperature and requires additional processing stepsfor employing in an electrolyte. Graphite electrodes have a fragilestructure and, until the invention of the electrolyte described herein,have required the use of EC for forming the SEI layer without damagingthe graphite structure. By contrast, hard carbon negative electrodes arenot as easily broken and therefore can use solvents other than EC toform the SEI layer. However, while hard carbon has a higher capacitythan graphite, it can absorb a lot of moisture and has a largeirreversible capacity, making graphite a much more desirable electrodematerial than hard carbon. Lithium metal does not require EC to form anSEI layer, but is useful only for a primary battery, not rechargeable.Vinylene carbonate (VC) and vinyl ethylene carbonate (VEC) can aid increating an SEI layer, but can only be used in quantities up to about 3%because an excess of these solvents creates degradation at the positiveelectrode; with this small quantity of SEI-forming solvent, only a thinSEI layer is created, with all of the VC or VEC consumed during thefirst charging cycle; therefore, another SEI-forming component such asEC must be added.

SUMMARY

The electrolyte of the present invention comprises a salt or mixture ofsalts comprising lithium bis(oxalato) borate (LiBOB) in a lactonesolvent or mixture of lactone solvents, preferably gamma-butyrolactone(GBL), combined with a low viscosity solvent or mixture of low viscositysolvents, and preferably does not contain a solvent that is solid atroom temperature, such as ethylene carbonate (EC). This inventiveelectrolyte is useful in primary and secondary batteries, and isespecially suitable for a lithium ion battery having a graphite negativeelectrode, forming a functional SEI layer that does not readilydecompose.

LiBOB is more soluble in lactone solvents, such as gamma-butyrolactone(GBL), than in commonly used carbonate solvents, such as ethylenecarbonate (EC) and propylene carbonate (PC). Using a lactone solvent todissolve LiBOB electrolyte produces a high salt concentrationelectrolyte, greatly improving conductivity as compared with using acarbonate solvent.

This electrolyte system has a wide operating temperature range andtherefore can be safely used in many applications, including satellitesand implantable medical devices. For example, a high temperaturesterilization process could not be used for many electrolytes; the saltLiPF₆ decomposes at about 80° C., and DEC boils at about 126° C. Bycontrast, LiBOB is stable at 300° C., and GBL boils at about 206° C.,making this combination ideal for high temperature sterilization. At theother temperature extreme, EC has poor low temperature performance dueto its high freezing point of around 37-39° C., making it very viscousat low temperatures, and therefore less desirable for applications inwhich low temperature operation is important.

Furthermore, in the case of a leak, unlike fluorine-containing saltssuch as LiPF₆, LiBOB does not form HF when mixed with bodily fluid, andis therefore safer than LiPF₆. While LiBF₄ decomposes at a lower ratethan LiPF₆ and is therefore slower to form HF, it has lower conductivitythan LiPF₆ due to its lower dissociation.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 a is the chemical formula of the salt in the electrolyte of thepresent invention and FIG. 1 b is the chemical formula of GBL.

FIG. 2 shows the test set up for single cell tests described herein.

FIG. 3 is a graph of the comparison of rate properties of an electrolytecomprising LiBOB in different solvents.

FIG. 4 is a graph showing capacity retention data of cells usingelectrolytes of the present invention compared with cells usingelectrolytes containing EC.

FIG. 5 is a graph showing capacity cycle life data of cells usingelectrolytes of the present invention compared with cells usingelectrolytes containing EC.

DETAILED DESCRIPTION

The following text describes the preferred mode presently contemplatedfor carrying out the invention and is not intended to describe allpossible modifications and variations consistent with the spirit andpurpose of the invention. The scope of the invention should bedetermined with reference to the claims.

The electrolyte of the present invention is a solution of LiBOB salt, alow viscosity solvent, and a lactone, for example, gamma-butyrolactone(GBL). FIG. 1 a is the chemical formula of the LiBOB salt in theelectrolyte of the present invention, and FIG. 1 b is the chemicalformula of GBL.

A typical electrolyte comprises 1.2-M LiPF₆ in EC:DEC. The viscosity ofEC is about 1.86 centipoise (cP) at 40° C. GBL has a viscosity of about1.7 cP at room temperature. A low viscosity solvent is one that willlower the overall viscosity of the electrolyte comprising LiBOB and GBLand is therefore less viscous than GBL. Therefore, the low viscositysolvent itself has a viscosity of less than about 1.7 cP and morepreferably less than about 1 cP. Low viscosity solvents can be chosenfrom among the following: nitrites such as acetonitrile, ether such asdimethyl ether (DME) or tetrahydrofuran (THF), linear carbonates such asdiethyl carbonate (DEC) and methyl ethyl carbonate (MEC), and linearesters such as propyl acetate (PA) and methyl acetate (MA). An advantageof using a noncarbonate low viscosity electrolyte is that carbonatestend generate CO₂ gas when decomposing, which can cause the battery toswell.

FIG. 2 shows the test set up for tests carried out to ascertain rate andcycle life properties of the electrolyte of the present invention usinga single cell 20. A negative electrode 22 comprising graphite activematerial on a copper substrate is separated from a positive electrode 24comprising a positive active material on an aluminum substrate by aseparator 26. The electrodes 22 and 24, separator 26, and electrolyte 28are enclosed by an aluminum foil bag 29 to form cell 20.

FIG. 3 is a graph of the comparison of rate properties of an electrolytecomprising LiBOB in different solvents at two different discharge rates.LiBOB is much more soluble in GBL than in EC. 0.5-M LiBOB in 3:7 EC:DEC,which is a commonly-used solvent combination, is a saturated solution,whereas 1.2-M LiBOB in 3:7 GBL:DEC is close to saturated. Because somuch more LiBOB salt can dissolve in 3:7 GBL:DEC than in 3:7 EC:DEC, theconductivity can be made much higher for 3:7 GBL:DEC than for 3:7EC:DEC. This increase in conductivity reduces polarization, which leadsto greater discharge capacity. Therefore, GBL mixed with a low viscositysolvent enables LiBOB, which is inherently safer than LiPF₆ and LiBF₄,to be used where large discharge capacity is required.

FIG. 4 is a graph showing capacity retention of single cells usingelectrolytes of the present invention compared with cells usingelectrolytes containing EC, using a graphite negative electrode 22 andLiNi_(0.8)Co_(0.15)Al_(0.05)O₂ positive electrode 24 using the setup asshown in FIG. 2. Both of the inventive solvent combinations shown havegood capacity retention, with LiBOB in GBL/PA somewhat better than LiBOBin GBL/DEC.

FIG. 5 is a graph showing cycle life data of spiral wound batteriesusing the same electrolytes of the present invention as in FIG. 4compared with cells using electrolytes containing EC. The tests weredone using a graphite negative electrode 22 and LiCoO₂ positiveelectrode 24. In this test, the starting discharge capacity at the firstcycle is less important than the slope of the curve, which is ideallyzero. Both of the present invention solvent combinations were shown tohave good cycle life, comparable to or better than those containing EC.Therefore, the inventors have discovered that EC is not a necessaryelectrolyte component for forming an SEI layer on graphite, and that thecombination of LiBOB with GBL and a low viscosity solvent such as PA orDEC is suitable for use as a battery electrolyte.

An electrolyte of the present invention may be made simply by combininga measured mass of GBL with a measured mass of low viscosity solvent,such as PA, then dissolving in a measured mass of LiBOB salt. The entireprocess may be completed at room temperature, or even lower, if desired.

By contrast, an electrolyte containing EC requires first melting the ECat elevated temperature such as in an oven in a dry environment, whichcan take about 5 hours for a 1-L bottle. Then the melted EC must betransferred immediately to an argon box and accurately weighed. Then itmust be quickly combined with one or more additional weighed solvents,and then the measured mass salt dissolved before the EC begins torecrystallize. Because of the additional steps of melting the EC and therequired use of heat, manufacturing an EC-containing electrolyte is moreexpensive than manufacturing the electrolyte of the present invention.Scaling up the EC-containing electrolyte manufacturing process iscostly, requiring expensive equipment.

A battery of the present invention may be made by housing an electrodeassembly in a battery case and inserting an electrolyte as describedherein into the case, wherein the electrolyte comprises LiBOB salt in acombined solvent of lactone, preferably GBL, and a low viscositysolvent. The negative electrode of the electrode assembly may comprisegraphite, hard carbon, lithium, lithium alloy, SiO, Si, SnO, Sn, and/orany other negative electrode material known in the art. The negativeelectrode may further comprise a negative electrode substrate made ofcopper, titanium, nickel, or stainless steel. The positive electrode maycomprise a carbon fluoride, a cobalt oxide, a nickel oxide, a nickelcobalt oxide, a manganese oxide, a manganese cobalt oxide, a nickelcobalt manganese oxide, silver vanadium oxide (SVO), a lithium titaniumoxide, iodine, and/or any other positive electrode material known in theart. The positive electrode may further comprise a positive electrodesubstrate made of aluminum, nickel, titanium, or stainless steel. Thebattery may be a primary or secondary (rechargeable) battery. If it is arechargeable battery, it may be a lithium ion battery having a liquidelectrolyte, or may have a polymer electrolyte, which could be a gel ora solid in combination with a liquid electrolyte. For an implantablemedical device, the device housing and/or the battery, which may behoused within the device housing, is hermetically sealed. For a medicaldevice requiring high temperature sterilization or for other hightemperature applications, the low viscosity solvent is preferably chosento have a high boiling point, such as greater than 126° C.

While the invention herein disclosed has been described by means ofspecific embodiments and applications thereof, numerous modificationsand variations could be made thereto by those skilled in the art withoutdeparting from the scope of the invention set forth in the claims.Furthermore, various aspects of the invention may be used in otherapplications than those for which they were specifically describedherein.

1-50. (cancelled).
 51. An electrolyte consisting of: one or more salts,including LiBOB; one or more lactones; and one or more low viscositysolvents; wherein the electrolyte is capable of forming an effective SEIlayer on an electrode.
 52. The electrolyte of claim 51 wherein the oneor more salts does not contain fluorine.
 53. The electrolyte of claim 51wherein the one or more salts consists of LiBOB.
 54. The electrolyte ofclaim 51 wherein the one or more lactones includes gamma-butyrolactone.55. The electrolyte of claim 51 wherein the one or more lactonesconsists of gamma-butyrolactone.
 56. The electrolyte of claim 51 whereinthe one or more low viscosity solvents includes a linear ester.
 57. Theelectrolyte of claim 51 wherein the one or more low viscosity solventsconsists of a linear ester.
 58. The electrolyte of claim 51 wherein theone or more low viscosity solvents includes propyl acetate.
 59. Theelectrolyte of claim 51 wherein the one or more low viscosity solventsconsists of propyl acetate.
 60. The electrolyte of claim 51 wherein theone or more low viscosity solvents includes methyl acetate.
 61. Theelectrolyte of claim 51 wherein the one or more low viscosity solventsincludes a nitrile.
 62. The electrolyte of claim 51 wherein the one ormore low viscosity solvents includes acetonitrile.
 63. The electrolyteof claim 51 wherein the one or more low viscosity solvents includes anether.
 64. The electrolyte of claim 51 wherein the one or more lowviscosity solvents includes dimethyl ether.
 65. The electrolyte of claim51 wherein the one or more low viscosity solvents includes a linearcarbonate.
 66. The electrolyte of claim 51 wherein the one or more lowviscosity solvents includes diethyl carbonate.
 67. The electrolyte ofclaim 51 wherein the one or more low viscosity solvents includes methylethyl carbonate.
 68. The electrolyte of claim 51 wherein the one or morelow viscosity solvents includes tetrahydrofuran.
 69. The electrolyte ofclaim 51 wherein the concentration of LiBOB is greater than 0.5 M. 70.The electrolyte of claim 51 wherein the concentration of LiBOB isgreater than 1.0 M.
 71. The electrolyte of claim 51 wherein theconcentration of LiBOB is about 1.2 M.
 72. A lithium battery comprising:a battery case; an electrode assembly housed in the case; and anelectrolyte in the case, the electrolyte including one or more lactones,one or more low viscosity solvents, one or more salts, and being capableof forming an effective SEI layer on an electrode; wherein the one ormore salts includes LiBOB.
 73. The battery of claim 72 wherein thebattery is rechargeable.
 74. The battery of claim 72 wherein the batteryis a lithium ion battery.
 75. The battery of claim 72 wherein thebattery is a lithium polymer battery.
 76. The battery of claim 72wherein the electrode assembly includes a graphite negative electrode.77. The battery of claim 72 wherein the one or more lactones includesgamma-butyrolactone.
 78. The battery of claim 72 wherein the one or morelow viscosity solvents includes a linear ester.
 79. The battery of claim72 wherein the one or more low viscosity solvents includes propylacetate.
 80. The battery of claim 72 wherein the one or more lowviscosity solvents includes methyl acetate.
 81. The battery of claim 72wherein the one or more low viscosity solvents includes a nitrile. 82.The battery of claim 72 wherein the one or more low viscosity solventsincludes acetonitrile.
 83. The battery of claim 72 wherein the one ormore low viscosity solvents includes an ether.
 84. The battery of claim72 wherein the one or more low viscosity solvents includes dimethylether.
 85. The battery of claim 72 wherein the one or more low viscositysolvents includes a linear carbonate.
 86. The battery of claim 72wherein the one or more low viscosity solvents includes diethylcarbonate.
 87. The battery of claim 72 wherein the one or more lowviscosity solvents includes methyl ethyl carbonate.
 88. The battery ofclaim 72 wherein the one or more low viscosity solvents includestetrahydrofuran.
 89. The battery of claim 72 wherein the concentrationof LiBOB is greater than 0.5 M.
 90. The battery of claim 72 wherein theconcentration of LiBOB is greater than 1.0 M.
 91. The battery of claim72 wherein the concentration of LiBOB is about 1.2 M.
 92. An implantablemedical device including the battery of claim
 72. 93. A method formaking a battery comprising the steps of: providing a battery case;housing an electrode assembly within the battery case; and inserting anelectrolyte into the battery case, the electrolyte including one or morelactones, one or more low viscosity solvents, one or more salts, andbeing capable of forming an effective SEI layer on an electrode; whereinthe one or more salts includes LiBOB.